For example, a Mach-Zehnder optical modulator used in a high-speed long-distance optical communication system employs a bias tee. The bias tee is used for applying a DC (direct current) voltage (offset voltage) to a high-frequency RF (radio frequency) signal line, and is usually housed in an housing as a bias tee circuit device together with an optical modulation device.
An example of the optical modulation device includes a Mach-Zehnder optical waveguide and progressive wave electrodes (coplanar electrical waveguide: CPW) for controlling the relative phase of light that propagates through each of the Mach-Zehnder arms, formed on a substrate having an electro-optical effect. In this optical modulation device, a modulation signal voltage applied to the progressive wave electrodes controls the index of refraction of each arm and changes the difference in length of the light paths between the two arms, thereby realizing optical modulation.
Here, obtaining an appropriate optical modulation signal by use of this optical modulation device requires application of an RF modulation signal having an appropriate voltage and a DC bias voltage (operation point control) appropriate for controlling the relative phase shift between the two arms to the MZ arm section. In other words, it is necessary to apply a high-frequency RF signal, which is a modulation signal, and also a superimposed DC voltage to the CPW electrodes formed in the modulator. The bias tee circuit element is used for applying a high-frequency RF signal, which is a modulation signal, and also a superimposed DC voltage to the CPW electrodes as described above.
A documented example of the known art related to the present invention is described in Japanese Laid-open Patent Publication No. 2007-010942.